1. Field of the Invention
The field of the present invention relates to wireless communication and, more particularly, to communication protocols for control traffic in a wireless communication system.
2. Description of Related Art
A mobile communication system may generally comprise a set of xe2x80x9cuser stationsxe2x80x9d, typically mobile and the endpoints of a communication path, and a set of xe2x80x9cbase stationsxe2x80x9d, typically stationary and the intermediaries by which a communication path to a user station may be established or maintained. A group of base stations may be connected to a base station controller, or a cluster controller, which can in turn be connected to a local public telephone network through, for example, a mobile switching center.
It is generally desirable in a mobile communication system to achieve the greatest possible user traffic capacity at a base station, so that fewer base stations need to be deployed in order to serve user demands. One technique used to allow a base station to communicate with multiple user stations is use of time division multiple access (TDMA). In a particular TDMA system, for example, a time frame is divided into a plurality of smaller time units, or time slots, and transmissions from the base station and from the user stations are separated in time so as to avoid collisions. In addition to separating transmissions in time, transmissions may also be distinguished by using different assigned frequencies, thereby resulting in a frequency division multiple access (FDMA) system. Furthermore, transmissions may be encoded using spread spectrum techniques, and different cells in a mobile communication system may be assigned different spread spectrum codes, thereby differentiating transmissions through code division multiple access (CDMA).
Generally, in order to carry out communication between a base station and a user station, a communication link must first be established. Establishment of the communication link can be difficult in a spread-spectrum communication system, due to the length of time typically required to synchronize the transmitter and the receiver. Establishment of the communication link and/or handing off can be more difficult in a TDMA system in which spread spectrum is used, due to the amount of time usually necessary to synchronize the transmitter and receiver, especially where the amount of time available for synchronization within a user station""s time slot is relatively brief.
Within a mobile communication system, a protocol generally defines how communication is to be initially established between a base station and a user station. The protocol may further define when and how a handoff may be conducted as a user station leaves the service area or xe2x80x9ccellxe2x80x9d of one base station and enters the service area of another base station. Messages exchanged between a base station and user station for the purposes of establishing or maintaining a connection, or for handing off communication, generally can be referred to as control traffic or signaling traffic. Messages carrying data to be conveyed between the endpoints of a call are generally referred to as bearer traffic messages.
Initial communication between a user station and a base station can be established either when the user station seeks to initiate communication with a base station (for example, attempting to initiate a telephone call), or when the base station attempts to complete a call to the user station (for example, where the user station is paged). In many conventional mobile communication systems, a dedicated control channel is used to assist mobile stations in establishing communication. According to this technique, the mobile station first communicates over the control channel when establishing communication. The base station then assigns to the mobile station a xe2x80x9cpermanentxe2x80x9d communication channel for exchanging bearer traffic messages.
In at least one mobile communication system, however, a user station can establish initial communication using the same channel used for transmitting bearer traffic. For example, a system in which a user station can establish communication by exchanging control traffic messages in a particular communication channel (e.g., a time slot of a time frame), and thereafter use the same channel (time slot) for bearer traffic, is described in U.S. patent application Ser. No. 08/284,053 filed Aug. 1, 1994, which is assigned to the assignee of the present invention, and hereby incorporated by reference as if set forth fully herein.
The exchange of control traffic messages may also occur during a handoff of a user station from one base station to another, usually as the user station moves between service areas. Typically, in the large majority of conventional mobile communication systems, handoffs are carried out under the direction of the base station and/or a mobility control center connected to the base station. When a communication link starts to break down, the base station requests a transfer of an ongoing call to a nearby base station, which becomes the target for handoff. The target base station may be selected according to criteria developed at the base station, the user station, or both. A control channel (which may be the same dedicated control channel as used for establishing communication, where provided) may be used for the purpose of assisting the mobile station with the handoff.
In some mobile communication systems, the user station plays a larger role in handoff. An example of such a system is generally described in U.S. patent application Ser. No. 08/284,053, previously incorporated herein by reference. In at least one embodiment disclosed therein, the user station not only determines when to hand off, but also takes steps to initiate a hand off from its current base station to a different base station.
It is generally desirable in mobile communication systems to allow the rapid establishment of communication links between mobile stations and base stations, and rapid handoff between base stations, without errors and without inadvertently dropping the call or losing a communication link. This type of capability would tend to imply the need for devoting potentially significant resources (i.e., communication channels and processing speed and power) to handle link establishment and handoff. Because the communication environment can be unstable and multiple users may need to be serviced at the same time, a mobile communication system is preferably capable of handling multiple service requests for link establishment or handoff, and doing so quickly and without errors or dropped calls.
At the same time, resources available for handling control traffic messages are usually limited, sometimes severely so, in part because control traffic resources generally must compete against bearer traffic resources. Thus, resources dedicated to control traffic reduce the overall resources available for handling data or bearer traffic, and vice versa. By setting aside resources (such as a dedicated control channel or multiple such channels) for servicing control traffic demands, the base station""s user capacity can be adversely impacted. As a result, a greater number of base stations may need to be deployed to service a given number of expected users.
It would therefore be advantageous to provide a communication system having a rapid and reliable means for establishing a communication link between a base station and a user station. It would further be advantageous to provide a communication protocol enabling rapid handoffs and control traffic functions, and which is particularly suited to use in a time division multiple access environment. It would further be advantageous to provide a communication protocol having a fast handoff and control traffic capability well suited to the demands of spread spectrum communication.
In one aspect of the present invention, a method and system for handing off communication between base stations in a mobile communication system is provided. In a preferred embodiment of the invention, a mobile station communicates with a base station using a time division multiple access (TDMA) and/or time division duplex (TDD) technique. In such an embodiment, a continuous sequence of time frames is generated, with each time frame comprising a plurality of time slots. The base station can communicate with a plurality of user stations (some or all of which may be mobile stations), one in each time slot. A mobile station desiring to hand off exchanges a plurality of control traffic messages with a second base station to establish communication in a different time slot with the second base station. The mobile station then releases the communication channel with the first base station and requests, through the second base station, the transfer of the call to the second base station.
In a preferred embodiment of the present invention, a mobile station transmits and/or receives a plurality of control traffic messages in multiple time slots of one or more time frames with the second (target) base station while in the process of handing off communication to the target base station, or performing other control traffic signaling. The second base station provides an indication to the mobile station of the next available time slot for control traffic, and, if desired, can temporarily assign additional time slots to the mobile station during handoff, or other control traffic signaling.
In another aspect of the present invention, a method and system for establishing communication and handing off communication in a TDMA and/or TDD communication is provided. In one embodiment, the base station transmits a general poll message in each available time slot to indicate availability of the time slot. To establish communication in an available time slot, a user station responds to the general poll message with a general poll response. The base station then follows with a specific poll message. The user station responds with a specific poll response. Normal traffic communication may thereafter be conducted over an established communication link. During normal traffic communication, in one embodiment, each user station transmits information to the base station during an initial portion of an assigned time slot, and each user station receives information from the base station during a latter portion of the same assigned time slot.
Handover between base stations may be carried out by establishing a new communication link with a new base station, while maintaining an old communication link with an original base station until the new communication link is fully established. The new communication link may be established in the same manner as the original linkxe2x80x94that is, by using the same handshaking technique involving a general poll, general response, specific poll, and specific response messages.
In another aspect of the invention, a slot pointer information element within a general polling message provides an indication of the location of the next available time slot for communication. The slot pointer may be a numerical value relative to the current time slot. As part of a specific polling message, the slot pointer information element provides an assignment of the time slot channel to be used for future communication by the user station presently in the process of establishing communication. The slot pointer may be used to perform rapid handover by allowing the use of multiple time slots within a time frame for control traffic.
In another embodiment, virtual time slots are defined as part of the timing structure. As used herein, a virtual time slot is generally a time slot assigned to the same user station with two transmission intervals non-adjacent in time. For example, a virtual time slot may be a time slot in which a forward link transmission and a reverse link transmission for a particular user station are separated by transmissions to or from one or more other user stations. In a preferred system in which each physical time slot has a user transmission interval and a base transmission interval, a user station may therefore transmit a user message to the base station during a user transmission interval of a first physical time slot, and receive a base message from the base station during a base transmission interval of a second, subsequent physical time slot. In a particular embodiment, a virtual slot field in the header of the general polling message indicates whether or not virtual time slots are provided, thereby enabling operation in either of two modes, one using virtual time slots and the other not using virtual time slots.